In-Mold Assembly: A New Approach to Assembly Automation

Students: A. Ananthanarayanan, W. Bejgerowski, D. Mueller, G. Ramu, P. Ward Advisor: S.K. Gupta

Sponsors: NSF and ARO MURI

•  We were the first research group to successfully realize mesoscale revolute joint using in-mold assembly

•  25% radial support found to be optimum for mold geometry and ABS/LDPE combination

New design enabled by in-mold assembly Consists of 5 pieces and no assembly operation

Traditional design created by machining and manual assembly Consists of 11 parts and 10 assembly operations

Goals •  Develop mold design templates to develop

mesoscale joints •  Develop model to estimate deformation of

premolded components and alternate ways to control it

•  Develop an understanding of in-mold assembly clearances

•  Develop design templates to embed electronics and actuators in mold

•  Develop models to understand heat dissipation of actuators embedded in polymers

Compliant Clip

Prismatic joint Universal Joint

Spherical joint

Applications

•  AML has built several versions of flapping wing MAVs using advances in injection molding. Molded drive mechanism converts rotary motor motion to flapping action for wings

•  In-mold assembly methods used to –  Automate assembly process –  Eliminate fasteners –  Decrease weight

Molded drive mechanism frame for Small Bird

Attributes of different MAVs

built at AML

SMA actuated robot suitable for Neurosurgery

Process Characterization and Modeling Flapping Wing MAV

Miniature Robot

Flapping wing MAV Drive Mechanism

•  Shape memory alloy (SMA) actuated robot developed by AML in collaboration with Robotics Automation Manipulation and Sensing (RAMS) Lab

•  In-mold assembly methods used to –  Downscale overall robot size –  Significantly reduce part count –  Eliminate fasteners

Part comes out of the mold fully assembled

This design contains parts whose largest dimension is less than 2 mm

Small parts and complex geometry make it very difficult to assemble this MAV swashplate

Capabilities

Second stage Injection with supported premolded components

First stage part

(ABS)

Second stage part

(LDPE) Plastic deformation of

premolded components Bent pins due

to second stage injection

Gate

First stage part (ABS),

pin diameter: 0.8 mm

Second stage part (LDPE)

Part with 0o

Orientation Part with 90o

Orientation

Weld-lines

Gate 1

Gate 2

First stage part

Second stage part

0º Orientation

90º Orientation

Weld-line location

d

L1

L2

Gate 1 Gate 2

Second stage melt Premolded

component

•  Alternative filling strategy to inhibit plastic deformation of premolded component

•  Premolded component deformation dependent on temporal misalignment of gates

Unidirectional Filling for In-Mold Assembly of Mesoscale Revolute Joints

Bi-directional Filling for In-Mold Assembly of Mesoscale Revolute Joints

In-Mold Assembly Concept

Joint Clearances during In-Mold Assembly of Mesoscale Revolute Joints

Temporal misalignment of gates

Forces applied Second stage polymer melt

dp’ Le

•  Premolded component undergoes axial plastic deformation due to compressive force applied by second stage polymer melt forming assembly clearances

•  Change in diameter (Dv) of the premolded component found to be related to support cavity length (Lc)

Change in premolded component dimensions due to second stage melt flow

Small Bird Big Bird Big Bird with vision

Big Bird with folding wings

Overall Weight 12.8 g 35.0 g 42.2 g 36.9 g Wing Span 34.3 cm 57.2 cm 57.2 cm 57.2 cm

Flapping frequency 12.1 Hz 4.5 Hz 4.5 Hz 4.5 Hz Payload Capacity 2.5 g 12.0 g 4.8 g 10.0 g

Small Bird built at AML

25 DOF Hand In-mold assembled revolute joint

1st stage part

2nd stage part

Mesoscale in-mold assembly methods utilized to manufacture 25 DOF hand

•  We use thermally conductive polymer composites to create multi functional structures with embedded actuators –  Anchoring of the embedded actuator –  Dissipation of heat produced by the actuator

•  Coupled modeling approach: –  Polymer melt flow inside the mold to obtain

fiber orientations –  Orthotropic thermal conductivity models from

molding process to assess heat dissipation

•  Research results: –  40% reduction in the operating temperature of the embedded

actuator –  Polymers with k > 2 W/m-K do not require orthotropic thermal

conductivity modeling

Embedding Actuators

k(Θ)

k(y)

Side core used as a radial supports

Gate

Bi-directional filling Computational model for plastic deformation

Computational model for plastic deformation

Side Mold Cores

Top Mold Piece

3 Piece Middle Layer Assembly

Bottom Mold Piece

Mold assembly for drive mechanism Computational model for change in diameter

Support Cavity

Gate Lc

In-mold assembled gearbox

Coupled computational modeling approach

In-Mold Assembly: A New Approach to Assembly Automation